Several patents teach methods of charge direction for liquid electrophotographic toners. In general, liquid toners are attractive because they satisfy the need for smaller toner particles than dry powder electrophotographic methods can deliver. These liquid-dispersed particles, however, need to have charge more or less permanently attached to them or they will not be impelled to move across the development gap of the print engine to form the image desired.
Charging schemes for liquid electrophotography (LEP) have previously taken one of the following forms:
1. Loosely associated, non-specifically absorbed charges macromolecules: (lecithin; Gibson et al., U.S. Pat. No. 4,897,332). Non-specifically adsorbed macromolecules such as lecithin do not give either consistent or permanent charge to the toner. They also serve to increase average particle size which tends to reduce mobility relative to a smaller particle with the same surface charge.
2. Acid-base chemistry with the addition of carboxylates: (K. Pearlstine, I. Page and L. El-Sayed, Journal of Imaging Science, Vol. 35, No 1, January/February 1991, pp 55-58). The Pearlstine article discloses toner particles with carboxylic acid substituents as charge directors. The carboxylic acids are bound or associated with the toner particles via Van der Waals forces. This is a form of nonspecific adsorption, and is readily reversible. The carboxylic acid group can serve as an electron pair donor to an appropriately selected metal ion and thus provide charge direction for the toner particle. There is, however, a high probability that at least some of the total charge in the system is spread uniformly through the continuous phase and not localized on the toner particles.
3. Complexed metals: (8-hydroxyquinoline; Elmasry et al., U.S. Pat. No. 4,925,766. Beta-diketones; Lane, U.S. Pat. No. 5,028,508. Salicylates; Swidler, U.S. Pat. No. 5,045,425). These patents teach the use of more specific binding agents to complex the desired ion, however, they rely on mono or bidentate ligands and in general, weakly coordinating ligands such as salicylate, carboxylate, and phenol. Elmasry et al. teaches the inclusion of other complexing agents such as 8-hydroxyquinoline as monomers in the polymeric resin coating which are adsorbed onto the pigment colorant. Lane teaches the use of .beta.-diketones as bidentate ligands. Swidler teaches substituted salicylates as examples of bidentate ligand complexing agents. These ligands typically possess oxygen or nitrogen donor atoms to donate electron pairs into the coordination sphere of a metal ion. The oxygen donor sites are most often protonated, such as in the case of carboxylic acids or phenol. The donor sites may also be non-protonated, as in the case of some nitrogen donor atoms or beta-diketones.
In these prior art approaches, the coordinating functional group does not have a high affinity for the metal ion, and the formation constant for the metal/ligand complex is low. This means that at least some, if not a large proportion, of the metal ion will not be associated with the ligating groups. Instead, metal ions will be dispersed in the continuous phase rather than located on the toner particles, thus contributing to the overall conductivity of the dispersion and, due to the greater electrophoretic mobility of the dispersed metal ions, suppressing the migration of the toner particles in the electrical field.
The use of protonated binding sites on the ligating functional groups causes another problem. When the metal is bound into the binding site, the proton with its associated charge must go somewhere else. If it goes into the continuous phase it contributes to background conductivity and serves to suppress toner particle migration in the electrical field. Also, there is residual water in virtually all toners, and the proton may go into the residual water, and, if this happens, there may be inverse micro-micellar formation which can promote flocculation of the toner. This problem is one possible explanation for the observed flocculation phenomena in this type of toner.
4. Metal soap: (carboxylate complexes, specifically; Elmasry et al., U.S. Pat. No. 4,925,766). This charging scheme represents a subcategory of the complexed metals above, but is mentioned separately because the soaps may simply be added separately to the toner formulation in the hope that the long aliphatic chain portion of the soap will associate via Van der Waals forces with the resin coating of the toner particle.
All liquid toners have a need for charge on toner particles dispersed in hydrocarbon medium. Liquid-dispersed toner cannot be triboelectrically charged like dry powder toners and must instead have charge that is more or less permanently associated with the toner particle. The prior art provides this charge, but does so less efficiently than the present invention.